Voltage regulator



April 24, 1956 E. L. JOHNSON, JR 2,743,413

VOLTAGE REGULATOR Filed April 4, 1952 AMGIEN'T TEMPERATURE. COMPENSATING'fiHarzwsToa P AD CQMPENSATING BALLAST HERM 5T Qeam'm E CONSTANT VOLTAGEPic). 1 THEQMISTOQ BALLAST 2 my :zzss'mucz VOLTAGE REGULATION 3 ZONE PINVENTOR Emil L. Jolznsolglr'.

ATTORNEY United States Patent VOLTAGE REGULATOR Emil L. Johnson, In,Baltimore, Md., assignor' to Bendix Aviation Corporation, Baltimore;Md-., a corporation of Delaware Application April 4, 1952, Serial No.280,581 4 Claims; (Cl. 323-69) This invention relates generally tovoltage regulating apparatus and more particularly to a voltagevregulator embodying a special resistance device having; a. negativetemperature coefiicient ofv resistance and which, with associated.circuitry, provides simple and efi'ective regulation of a source ofvarying voltage.

Such special resistance devices are commonly called thermistors andnormally take the form of a sintered combination of ceramic materialsand various metallic oxides, the latter group of materials beinggenerally known as semi-conductors. Such units can easily be providedwith metallic electrodes, to which tinned copper leads can beconveniently soldered. Hence, there is available an element adapted for.ready insertionin conventional circuitry, and possessing specialelectrical characteristics which can be availed: of forpurposesofvoltage regulation.

It has been found that upon an increase in current flow through athermistor, a current value is reached wherein the thermistor begins toself-heat, i. e., the internal resistance heat-loss causes an elevationinthe internal temperature of the thermistor, thus causingthe resistanceof the thermistors to decrease in value, per mitting more current toflow and'hence further self-heating of the thermistor. A voltage-currentcurve for this region of self-heating will show a constant voltage dropacross the thermistor through a limited range in current values, thussuggesting the possibility of usinga ther-' mister for purposes ofvoltage regulation. If acurrent limiting resistor is placed in serieswith a thermistor across a. source of varying voltage, output leadsconnected across the thermistor will derive therefrom a substantiallyconstant voltage. Under' conditions of constant ambient temperature,such a system would give excellent results. However, normal field use ofvoltage regulating apparatus imposes variable: ambient temperatures uponthe system and since the resistance of the regulating thermistor is alsodependent upon ambient temperature, some means must be provided'tocompensate for such side-effects. It hasbeen found that a secondthermistor, locatedin the output-circuit, can be utilized for suchcompensation. This second thermistor will have to operate below theself-heating current range so-that it is responsive to'temperatureonlyand thus respond properly as a compensator,

Accordingly, it is an object ofthe invention toprovide an improvedvoltage regulatorembodying simple resistance elements and capable ofaccurate performance over a wide range in ambient temperatures.

The invention will be best understood, and its advantageous and novelfeatures readily ascertained, upon reference to the detailed descriptionset forth below, when taken in conjunction with the drawings, in which:

Figure 1 shows the improved voltage regulator in its most elementaryform, and

Figure 2 shows the current-voltage characteristic curve of theregulating thermistor embodied in Figure l, and

Figure 3 shows a series of current-voltage characteris 2,743,413Patented Apr. 24, 1956 tic curves of the regulating thermistor of Figure1 when same is subjected to variations in ambient temperature, and

Figure 4 shows a temperature measuring, circuit sinzodying the voltageregulator of Figure l and having means for compensating for the effectsof ambient tern"- perature changes.

Referring to Figure 1, there isshown a thermistor connectedin series.with a ballast resistance, there being an input voltage impressed acrossthe combined circuit. The voltage-current characteristic of thethermistor is generally shown in Figure 2' and inspection ofthisTcha-rac' teristic curve readily shows asubstantially constant voltage drop for a limited range of current flow. In this range is thevoltage regulation zone, and for the particular thermistor used, it isnecessary to have suificient ballast resistance to limit the currentflow to the aforementioned range during an excursion of input voltagethroughout the permissible deviation.

it was found that excellent results could be attained with thethermistor in the form of a thin rod having a diameter of .018 inch anda length of one and a half inches. Such a thermistor would have aresistance at plusv 30 degrees C. on theorder of 35,000'olnns and itstemperature coeflicient of resistance per degree C. at 20 degrees C. isminus 2.7%. The voltage regulationv zone was found to be within thecurrent range of 20' 50 mil liarnperes. Thus, a ballast resistance onthe order of 2,000 ohms was found tobe necessary to limit the'currentwithin the proper range for voltagercgulation.

In practice, with the circuit values as set forth above, the thermistorprovided a regulated voltage across its terminals of 40 voltsplus orminus 1 volt under a range in impressed voltage. of to 125 volts. For arange in impressed voltage of to volts, the regulated output voltage wasfound to be 40 volts plus or minusvonehalf volt, and for a total changeof 5 volts in supply, the regulator would be. accurate toone-quartervolt.

Now referring to Figure. 3, there is shown-a series of voltage-currentcharacteristic curves for the thermistor under conditions of varyingambient temperature. The current range for voltage regulation decreaseswith increasing temperature, as does the' voltage drop itself.Obviously, with such a condition existing, some means must be found tocompensatev for this temperature effect if the regulator is to performsatisfactorily in a measuring circuit. Furthermore, the ballastresistance must beselected to limit the current to its minimum range atmaxi mum temperatures-and for a range inambient temperature of minus30degrees F. to plus 1 10 degrees F., a value of 2,000 ohms as aforesaidwasadequate.

With the above in mind, and referring to Figure 4, therewill now bedescribed a temperature. measuring circuit embodyinga thermistor forvoltage. regulation. A source of alternating current 7 is connected.across the series. combination of thermistor 5 and ballast resistance 6.Connected across the thermistor 5 is the primary winding 8 of astep-down transformer having-its secondary. Winding 9 connected acrossthe potentiometer 11. Between the winding 9 and potentiometer l lthereis interposed a conventional rectifier 12'for converting the alternatingcurrent source to a direct current suitable for use in the measuringcircuit.

Connected across potentiometer 11 is a Wheatstone bridge measuringcircuit comprising potentiometer arm 13, fixed arm 14, and conditionsensing arm 15. The element 15 is a thermistor for measuring temperatureand may be located remotely from the remainder of the circuit. Theindicating diagonal of the bridge network comprises the microammeter 16and a compensating thermistor 17.

Both the condition sensing thermistor 15 and compensating thermistor 17are operated well below their self-heating current range so that theyrespond solely to changes in ambient temperature. Thus, if the ambienttemperature decreases, the regulated voltage supplied to thebridgecircuit by thermistor'S will increase. However, the compensatingthermistor 17 will undergo an increase in resistance, resulting in aconstant voltage drop across the microammeter for a given setting of thebridge. Should the ambient temperature increase, the action set forthabove would reverse, resulting in the same voltage drop across themicroammeter.

Obviously, if the regulator is to function properly, the thermistors 17and 5 should be subjected to the same, or substantially the same ambienttemperatures, or temperatures which are proportionally related to oneanother. When the entire regulator is located in a single, compacthousing, the ambient temperatures would ordinarily be substantially thesame for both 'thermistors. However, should the measuring circuit belocated at a point remote from the input circuit, then some means may benecessary to regulate the temperatures so that they are proportionallyrelated to one another. In either case, the thermal inertia of thethermistor 17 (which varies in relation to the diameter of a thermistorat a given length of the latter) should be properly correlated with thatof the thermistor 5.

Thus, by the use of two thermistors and a ballast resistor, a fullytemperature compensated source of regulated voltage is available for usein measuring circuits. The thermistor elements are inexpensive, areunaffected by ambient humidity or pressure, and have an indefinite life,all of which are quite beneficial for the intended use.

What is claimed and desired to be secured by United States LettersPatent is:

1. In a condition-responsive electrical network, a measuring oranalogous circuit, a transformer having a primary winding adapted toreceive load current from a source of supply and a secondary windingconnected to said measuring circuit, means connected between saidprimary winding and said source for compensating for limited changes inload current comprising a resistor having a negative temperaturecoefiicient of resistance and a voltage-drop-versus currentcharacteristic such that a subtially constant voltage drop existsthereacross throughout a given range of current flow and a currentlimiting resistor coacting with said first-named resistor formaintaining the current flow through the latter within said range, and'atemperature-compensating resistor connected in said measuring circuitfunctioning to automatically compensate for changes in transformeroutput voltage due to the efiects of ambient temperature on saidfirst-named resistor, said temperature compensating resistor also havinga negative temperature coefiicient of resistance and being operatedbelow its self-heating current range so as to respond solely to changesin ambient temperature.

2. In a condition-responsive electrical network, a measuring oranalogous circuit, a transformer having a primary winding receiving loadcurrentfrom a source of supply and a secondary winding connected to saidmeasuring circuit, means connected between said primary winding and saidsource for compensating for limited changes in load current comprising athermistor having a voltagedrop-versus current characteristic such thata substantially constant voltage drop exists thereacross throughout agiven range of current flow and a current-limiting resistor coactingwith said thermistor for maintaining the current flow through the latterwithin said range, and a temperature-compensating thermistor connectedin said measuring circuit functioning to automatically compensate forchanges in transformer output voltage due to the effects of ambienttemperature on said first-named thermistor, saidtemperature-compensating thermistor being operated below itsself-heating current range so as to respond solely to changes in ambienttemperature.

3. In a condition-responsive electrical network, a source of supply ofload current, means for compensating for limited changes in load currentover a given range of current flow comprising a thermistor connectedacross the supply line and having a voltage-drop-versus-currentcharacteristic such that a substantially constant voltage drop existsacross the thermistor throughout said range and a current-limitingresistor coacting with said thermistor for maintaining current flowthrough the latter within said range, and a temperature-compensatingthermistor connected into said network functioning to automaticallycompensate for changes in the voltage drop across said first-namedthermistor due to the effects of ambient temperature on said latterthermistor, said temperaturecompensating thermistor being operated belowits selfheating current range so as to respond solely to changes inambient temperature and said temperature-compem' sating thermistor beingsubjected to ambient temperatures which are substantially the same orare proportionally related to the ambient temperatures to which saidfirstnamed thermistor is subjected.

4. In a condition-responsive electrical network, in combination, aWheatstone bridge circuit having a condition-responsive resistor in oneleg thereof, a source of electric potential connected to the inputcircuit of said bridge, a thermistor connected across the supply linesof said source and having a voltage-drop-versus-current characteristicsuch that a substantially constant voltage drop exists across saidthermistor throughout a given range of current flow through the latterand means for maintaining the value of current flow through saidthermistor within said range, and meansfor automatically compensatingfor the'effects of ambient temperature changes on said thermistorincluding a second thermistor connected in the bridge circuit, saidtemperature-compensating thermistor being operated below itsself-heating current range so as to respond solely to changes in ambienttemperature and said compensating thermistor being subjected to ambienttemperatures which are substantially the same or are proportionallyrelated to the ambient temperatures to which said first-named thermistoris subjected.

References Cited in the file of this patent UNITED STATES PATENTS

